2014年度 惑星大気・電離圏セミナー

■場所 :A棟5F会議室 (変更の場合は赤字)
■時間 :水曜日16:30- (変更の場合は赤字)

開催日時・場所 発表者 (所属・身分)

2/25(Wed) 16:30 5F conference room Atsushi Yamazaki
Scientific topic of stellar observations by EUV spectroscopy of Hisaki ひさき衛星のEUV分光観測による恒星観測に期待する科学トピック  
ひさき衛星では較正観測として恒星を観測しました。観測結果、恒星EUVス ペクトルは予想外な特徴が見えてきました。系外惑星系やその中心星の観測から 惑星環境の理解につながる科学トピックを紹介します。

2/4(Wed) 15:30 5F conference room Ko-ichiro Sugiyama
Development of a moist convection model (Progress report)  
We have been developing a moist convection model "deepconv", which is a numerical fluid dynamical model that solves dynamical and thermodynamical equations and some physical processes such as subgrid turbulence and cloud microphysics. Recently we use this model to examine the characteristics of convective motion in Venus and Jovian planets atmospheres. In the seminar, we present some topics about improvement of our model and convective features obtained by our numerical simulations.

1/28(Wed) 16:30 5F conference room Takeshi Imamura
Classification of waves in the Venusian atmosphere and their roles in the general circulation  
Various types of atmospheric waves that are known to exist in the Venusian atmosphere are reviewed. Small-scale gravity waves transport zonal momentum principally in the vertical direction. Since their phase velocities are centered around the zonal wind velocity at the atmospheric region where they are generated, the momentum transport by those waves cannot induce flows faster than the flows in their source region. Because of this nature, gravity waves are not expected to induce the super-rotation; the zonal drag force caused by gravity waves should be balanced by other acceleration mechanism. Although thermal tides are a type of gravity waves and also propagate vertically, they can accelerate the upper atmosphere in the direction of the super-rotation because they are forced externally by the zonally-traveling solar heating. Kelvin waves observed at the cloud top might be generated as a resonant mode of the whole atmosphere and can also help sustaining the super-rotation via vertical momentum transport. Other planetary-scale waves can accompany latitudinal fluxes of zonal momentum; equatorward transport of zonal momentum can sustain the super-rotation when combined with the Hadley circulation. Rossby waves generated through barotropic/baroclinic instabilities of the mid-latitude or polar jets are the candidates of such waves.

1/21(Wed) 16:30 5F conference room Takayuki Enomoto
Preparative works of near-future IR imaging-polarimetry observations with NIIHAMA and SOLAR-C on Haleakala  
To enable acquisition of infrared (IR) imaging-polarimetry data of Venus in the coming best season (May 2015, with the phase angle 〜80 degs), preparative works were done at the Institute for Astronomy (IfA), University of Hawaii, Maui. During the 3-month stay there, knowledge of infrared optics, polarization instrument, knowhow of instrumentations were obtained. Venusian clouds and hazes play an important roll in some aspects such as chemical process and radiation in its atmosphere. From recent ground-based observations, it is indicated that the optical thickness of the polar haze layer is now in decreasing phase. As the haze optical thickness decreases, observed polarization is now dominated by that of main cloud. Multi-wavelength polarimetry in IR (at phase angles 〜80 degs) has potential to determine cloud properties with single observation run unlike time-consuming optical observations. Once we know better about the cloud, precise determination of "thinning" haze should be possible. Following these works, more will be done in first 2 weeks in February 2015, so the preparation for May 2015 observations will be completed. Between February and May, computation codes will be developed and example model computations will be done to maximize the scientific results. 

1/14(Wed) 15:30 5F conference room Yumika Sakamoto
スポラディックE層の観測・研究は古くからなされており,その生成機構についてはwind-shear理論が一般に受け入れられている.しかし,この理論は電子密度の集積過程を説明するにとどまり,Es層付近の熱収支についてはほとんど情報を与えない.熱収支を議論する上で,電子温度は重要なパラメータの一つであるが,Es層内での正確な電子温度観測の例は極めて少ないうえに古いものが多く,かつ信頼性に乏しい.このように,スポラディックE層内の熱エネルギー収支を議論するために必要な電子温度情報は限られた報告例しか存在せず,信頼性のある観測データが待ち望まれている.また,一般にスポラディックE層の高度方向の幅は非常に狭いため,高速で飛行する観測ロケットによる詳細観測は一般に難しい.スポラディックE層内での電子密度観測については報告例が多数あるが,データ点数が粗く,詳細空間構造を議論する上で十分ではない.このため,高速でサンプリングが可能な測定器による観測が求められている. このような問題を解決すべく,下部電離圏に発生するスポラディックE層の空間構造解明を主目的として平成26年8月17日19時10分00秒(JST)に宇宙航空開発機構内之浦宇宙空間観測所より観測ロケット(S-520-29号機)が打ち上げられた.本研究の目的は観測ロケットに搭載されるラングミュアプローブがロケット軌道上で取得するデータを用いて,電離圏の高度90〜243 km領域での電子温度および電子密度を推定し,上昇時スポラディックE層内の詳細空間構造ならびに新たな解析方法を用いることにある. その結果,電圧掃引時間内に変化しているスポラディックE層内の電子密度・温度を,新たな解析方法を用いて正確に求めることに成功した.また,スポラディックE層の厚さと,ロケットスピンによる影響を確認することができた. 

12/10(Wed) 15:30 5F conference room Mayu Miyamoto
Paper introduction from "Gravity wave dynamics and effects in the middle atmosphere" by Fritts and Alexander (2003)  

12/3(Wed) 15:30 5F conference room Takumi Abe
Electron density irregularity in the ionosphere  
Electron density irregularities are observed to exist in various spatial scales in the ionosphere. Data from sounding rocket, satellite and the ground-based radar measurements have been used to investigate characteristic feature of the density irregularities, which ranges from large-scale (about 100 km) to the meter-size domain. S-520-27 sounding rocket experiment was conducted in July 2013 to investigate the electromagnetic coupling between E and F region of the ionosphere, focusing on the generation process of the medium-scale traveling ionospheric disturbance (MS-TID). In this experiment, measurements of neutral wind, thermal plasma, and electric/magnetic fields were carried out by science instruments onboard the rocket. Among them, the objective of Fixed Bias Probe (FBP) is to provide information on electron density irregularity with a spatial scale down to < 1 m. A result from the detailed analysis of the electron density irregularity measurement will be presented, with a particular interest in the altitude variation of its spectral behavior. General characteristics of the density irregularity varies with the altitude and spatial scale, which may suggest the different causes with altitude. Altitudinal variation of the spectral index will also be discussed.  

11/26(Wed) 16:30 5F conference room Hiroki Ando
Short Talk  

11/19(Wed) 16:30 5F conference room Ko-ichiro Sugiyama
Preliminary results of thermal convection in the Martian lower atmosphere using CReSS-Mars  
To support the activity of a Mars surface exploration program, we are now trying to apply a numerical meteorological model (CReSS, Tsuboki and Sakakibara, 2002) to Martian atmosphere. We introduce a radiative transfer scheme for Mars (Takahashi et al., 2006) to CReSS, and topography data and the values of physical constants are changed as to be suitable for Mars. Some preliminary numerical experiments to examine performance of CReSS-Mars are now progressing successfully. In this presentation, we show outline of CReSS-Mars and some remarkable results obtained in the experiments, such as the dependency on the mixing length included in the turbulence process.  

11/12(Wed) 15:30 5F conference room Miho Kanao
The study of the refracted aureole observed on the transit of Venus  
We know well that there is a thick cloud from 45 km to 70 km and haze layer in atmosphere of Venus. The distribution of the particles’ number densities is very interesting with the vertical and latitudinal dynamics through the layers in the relation with the super-rotation’s velocity. The transit has been a chance to observe the planetary atmosphere. With the high-developed optical instrument of Hinode / SOT (0.2”) and the nearest distance from the earth (0.3 AU), the vertical profile of the transmittance in the atmosphere could be lead from the images. The intensity of the arc refracted in the Venusian atmosphere is used. The refraction angle calculated with the number density profile ranges from about 1 deg (50 km height) to less than 0.0001 deg (100 km height) at the wavelength of CN band (388.35 nm), depending on the latitude. The geometry between the planets and the SUN, and the refraction angle could determine the source intensity of the refraction on the solar disk, respective to the arc altitude. Using the solar limb darkening profile from the Venus frames on the solar disk, we can remove the limb darkening effect of the refraction source from the arc intensity profile. The transmittance profile obtained from each image must be uniform. I will show you these method and the obtained results.  

11/5(Wed) 15:30 5F conference room Takehiko Satoh
Lightcurves of Venus  
While Venus orbits the sun, the solar phase angle (the sun - Venus - observer angle) changes. Because Venus is an inner planet, a full range of the solar phase angle (0 - 180 degs) is accessible from the earth. A lightcurve is obtained by measuring the brightness of Venus for variety of the solar phase angles. Near 0-deg phase angle, the lightcurve exhibits "glory" features characteristic for spherical particles in the main cloud layer. Near 180-deg phase angles, bumps in the lightcurve can be used to characterize the upper haze. In the seminar, Akatsuki (IR1+IR2) observations, VEx observations, as well as the ground-based observations are compared and discussed.  

10/29(Wed) 16:30 5F conference room Mayu Miyamoto
Radio holographic analysis of Venus' radio occultation data  
Gravity waves with vertical wavelengths from a few tens of meters to kilometers have been observed in the Earth’s atmosphere by radiosondes and radars. Also in the atmospheres of other planets, gravity waves are observed by various methods including radio occultation. The radio occultation method relies on the measurement of the frequency shift of the received signal caused by the bending of radio waves in the radial gradient of the refractive index in the atmosphere. The geometrical optics method has long been used for the analysis of radio occultation data. However, this method cannot disentangle multipath rays and vertical resolution is limited by the size of the Fresnel zone (about 1 km). Because of this limitation, only a limited part of the gravity wave spectrum has been covered, and thus the propagation and dissipation mechanisms of the gravity waves in other planets are poorly understood. Radio holographic methods have been proposed for processing of radio occultation signals in multipath regions and obtaining atmospheric profiles with high resolution. One of them is the Full Spectrum Inversion (FSI), which was recently applied to GPS occultation data of the Earth’s atmosphere. By applying this technique to Venus Express radio occultation data, we derived temperature profiles with high vertical resolution. In this presentation, the results among different latitudes and longitudes will be introduced.  

10/15(Wed) 16:30 5F conference room Yeon Joo Lee
Preliminary results of fluid dynamic model calculation of convective motion induced by solar heating at the Venus cloud top level.  
In the Venus atmosphere, about half of incoming solar radiation is absorbed at the upper cloud layer. An unknown UV absorber in the upper cloud layer is actually responsible to this strong absorption (Tomasko et al., 1980; Crisp et al., 1986; Lee et al., submitted), and induces significant diurnal variation in radiative energy balance at the cloud top altitude. Observed images from Venus Express and Pioneer Venus in UV range revealed that clear convective-like cells in the afternoon side of the planet (Rossow et al., 1980; Titov et al., 2012). These features are probably related to strong solar heating at the sub-solar point, rather than the influence from deeper level convection in the low and middle cloud layers (Imamura et al., 2014). In addition, a small difference in the cloud top structure may produce strong enough solar heating contrasts to trigger horizontal convection at this altitude (Toigo et al., 1994; Lee et al., submitted). In this study, we attempt to investigate the effect of solar heating at the cloud top on atmospheric dynamics, by incorporating radiative forcing calculated for the Venus cloud layer, and a nonlinear fluid dynamic model calculation (Cloud Resolving Storm Simulator, CReSS). We consider 2-dimensional domain, covering a full cloud deck from 40 km to 70 km. In this seminar we are going to show some preliminary results on the sensitivity study, in order to validate the conditions in the model calculation.  

10/1(Wed) 15:30 5F conference room Hiroki Ando
Venusian polar vortex reproduced in an Atmospheric General Circulation Model  
Venusian polar vortex has been observed by a lot of optical measurements but its vertical structure is not understood observationally and theoretically, and the mechanism to generate and maintain it is unknown. In this study we reproduce the polar vortex by using GCM (AFES) and investigate the generation mechanism. In addition we compared model results with the observational result obtained from radio occultation measurement. In this presentation we will show the preliminary result.  

9/24(Wed)16:30 5F conference room Takao Sato
Future plans for a better understanding of cloud top structure of Venus  
Our paper describing the cloud top structure of Venus revealed by the Subaru/COMICS mid-infrared images taken on 2007 is now in press in Icarus. Several follow-up observations using IRTF and Subaru and their data analyses are ongoing and planned. In this presentation, I would like to summarize our recent results and discuss future directions of this study.  

9/17(Wed)16:30 5F conference room Takeshi Imamura
Polar oval and low-latitude cloud features of Venus at visible wavelengths  
This is a follow-up report of my previous seminar talk given in June. Although Venus' clouds are considered to be almost featureless at visible wavelengths, analyses of Venus Express/VMC images in the visible channel showed existence of various contrast features such as the dark polar oval and mottled clouds in the low latitude. The morphologies of these features were studied by two undergraduate students of Univ. Tokyo. The preliminary results will be presented.  

8/20 (Wed) 15:30- 4F会議室 青木翔平
Seasonal variation of the HDO/H2O ratio in the atmosphere of Mars at the middle of spring and beginning of summer  
In this seminal, the seasonal variation of the HDO/H2O ratio caused by sublimation-condensation processes in a global view of the Martian water cycle will be present. The HDO/H2O ratio was retrieved from ground-based observations using high-dispersion echelle spectroscopy of the Infrared Camera and Spectrograph (IRCS) of the Subaru telescope. Coordinated joint observations were made by the Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX). The observations were performed during the middle of northern spring (Ls = 52°) and at the beginning of summer (Ls = 96°) in Mars Year 31. The retrieved latitudinal mean HDO/H2O ratios are 4.1 ± 1.4 (Ls = 52°) and 4.6 ± 0.7 (Ls = 96°) times larger than the terrestrial Vienna Standard Mean Ocean Water (VSMOW), which is consistent with previous observations. Conversely, the HDO/H2O ratio shows a large seasonal variation at high latitudes. The HDO/H2O ratio significantly increases from 2.4 ± 0.6 wrt VSMOW at Ls = 52° to 5.1 ± 0.7 wrt VSMOW at Ls = 96° over the polar region (70-80°N). This can be explained by preferential condensation of HDO vapor in the northern spring and sublimation of the seasonal polar cap in the northern summer. In addition, we investigated the geographical distribution of the HDO/H2O ratio over low latitudes at the northern spring in the longitudinal range between 220°W and 360°W, including different local times from 10 h to 17 h. We found the HDO/H2O ratio to be nearly constant within 5.1 ± 1.2 wrt VSMOW over the entire range. Our observations suggest that the HDO/H2O distribution in the spring and summer seasons is mainly controlled by condensation-induced fractionation between the northern polar cap and the atmosphere. In addition, some on-going works related MEX/PFS (water cycle, surface pressure, and CH4) will be present briefly.

7/23 (Wed) 15:30- 5F会議室 細内麻悠
Study of the Venus atmospheric waves by the ground infrared observations, and comparing with the UV images by VMC/Venus Express and Pirka telescope 地上赤外観測による金星大気波動現象の解析、VMC/Venus Expressおよび北大ピリカ 天文台の紫外画像との比較研究  
To figure out meteorological phenomenon in the Venus, we are searching wave phenomenon in the Venus. Especially, we want to know vertical changes of waves. We performed 4 times infrared spectroscopic measurements of the Venus’ days ide to find out atmospheric wave structures at 60 km in the cloud layer by q uantifying CO2 absorption. The apparent rotation periods of wave-like signat ure was derived and it may be interpreted as superposition of the mean zonal flow and waves such as the Kelvin and the Rossby. The last observation (performed in May 2014) has simultaneous observations by VMC/Venus Express and Pirka telescope(Hokkaido Univ.). Comparison among them will be the great step to know vertical structures of the Venus waves. 金星の大気現象を解明するため、大気波動現象、特にその鉛直構造を解明しようとし ている。 過去地上赤外観測を行い、60km付近の大気波動現象の研究を行ってきた。最新の観測 は、VMC/Venus Expressおよび北大ピリカ天文台と同日観測を行っており、その比較 から金星大気波動の鉛直構造を解明できることが期待されている。

7/16 (Wed) 16:30- 6F会議室 榎本孝之(D1)
Polarimetric investigation of Venusian clouds and hazes  
We performed polarimetry of Venus using HOPS (Hida Optical Polarimetry System) in visible wavelength range from 2012 to 2014. Through this study, we found a indication that the optical thickness of polar haze layers were as thin as the level of thiner period during Pioneer Venus mission and is now declining. To determine the amount of polar hazes, we need to estimate the optical properties of the main cloud for more precise estimation of decreasing hazes. In this seminar, I will show the brief reviews of polarimetry of Venus and new technique to estimate the cloud and haze properties using NIIHAMA IR camera attached to SOLARC telescope at the summit of Haleakala in Maui island.  

7/9 (Wed) 16:30- 5F会議室 阿部琢美 (准教授)
Sounding rocket experiment to investigate a spatial structure of sporadic E layer in the lower ionosphere  
ISAS is going to launch "S-520-29" sounding rocket in August 2014 to investigate a spatial structure of sporadic E layer in the lower ionosphere. Information on its density distribution in the horizontal direction is very limited while the vertical profile has been repeatedly measured by the sounding rocket and the radio sounding. In our experiment, a total of six science instruments are installed on the rocket, and will provide information on the characteristic structure of the plasma density distribution in both the horizontal and vertical direction. Magnesium ion imager onboard the rocket will detect emission from Magnesium ions due to resonant scattering of Sunlight, and it will be used to discuss the horizontal structure. Impedance probe and Langmuir probe will make in-situ observations along the rocket trajectory. It is possible to estimate altitudinal distribution of the electron density by analyzing intensity of LF/MF radio wave measured by receiver onboard the rocket. From these information, we will discuss 3-D structure of the sporadic E layer.  

7/2 (Wed) 15:30- 5F会議室 佐藤隆雄 (学振研究員)
Migrating tides in the Martian lower atmosphere deduced from MEX/PFS temperature data  
Thermal tides play an important role in determining the general circulation, thermal structure, and vertical coupling between the lower and the upper atmospheres of terrestrial planets. They are expected to be stronger on Mars than on Earth, because the Martian atmosphere is only about 0.6% as thick as the Earth's atmosphere. The characteristics of migrating tides have been investigated mainly by GCM studies. The comprehensive temperature observations by MGS/TES and MRO/MCS have been conducted from the sun-synchronous orbit, which means the characteristics of migrating tides still remain to be investigated. Planetary Fourier Spectrometer (PFS) onboard Mars Express (MEX) has observed atmospheric temperatures in the Martian lower atmosphere since the end of MY26. These data enable us to investigate the migrating tides in the lower atmosphere because the orbit of MEX is non sun-synchronous. In this presentation, I will briefly summarize the current knowledge of migrating tides in the Martian atmosphere and show some preliminary results.  

6/25 (Wed) 16:30- 5F会議室 宮本麻由 (D2)
Full Spectrum Inversion (FSI) of radio occultation signals for the Venus atmosphere  
The vertical distribution of the temperature, the electron density, the H2SO4 vapor density, and small-scale density fluctuation can be derives through the radio occultation experiment of the Venus atmosphere. The radio occultation experiment is based on the bending of radio waves caused by refractive index gradients in the atmosphere. A geometrical optics method is used for the analysis of the radio occultation data in general. However, this method cannot disentangle multipath rays and the vertical resolution is limited by the size of the Fresnel zone. Some high-resolution radio holographic methods have been proposed for processing of radio occultation signals in multipath regions. One of them is a Full Spectrum Inversion (FSI) technique for Earth’s atmosphere. We applied this technique to Venus Express radio occultation data. In this seminar, I’ll introduce FSI technique and show some early results.  

6/18 (Wed) 16:30- 5F会議室 金尾美穂(OD)
太陽観測衛星ひのでは地球周回軌道から2012年6月5日22:54から6月6日04:19(UT)に 金星の夜面を太陽面上に観測した。可視光磁場望遠鏡(SOT)は5日22:26から22:29までの間と 6日04:36から04:39の間には、太陽面外の金星リムに沿って、大気によって屈折したと思われる 光輪を観測した。 金星が太陽面上を通過中にG-band(430.5nm)において露光時間0.0512秒で連続的に観測された 画像を解析した。各々の画像において、0.067arcsec秒-1で移動する金星の位置を決定し、リムに おける明るさを緯度の関数として算出した。 リムにおいて、望遠鏡の回折の影響が最も少ないと考えられる透過率0.5の大気高度は、全緯度 方向に平均しておよそ90kmとヘイズ領域に相当していた。Venus Express衛星に搭載された SPICAV-IRとSPICAV-UVが観測した透過率0.5を高度と、今回のSOTの観測画像と比較し、 望遠鏡のピクセルサイズを有効数字5桁までの精度で決定した。 ピクセルスケールを用いて得られる、透過率T=0.5における大気高度の緯度分布は、ヘイズ領域の 雲粒分布が赤道付近で多く、極域では少ない事を示すものであった。 この研究結果を纏め、先行研究と比較し、紫外光における観測や、算出した消散係数の分布も合わせて 考察を行う。 The images on the transit of Venus on 5-6 June 2012 are acquired by the SOT onboard HINODE. We determined the position of Venus for each image and obtained the intensity at the limb of Venus. Comparing the observation by SPICAV-IR and SPICAV-UV, we defined the plate scale with high accuracy at the observation wavelength, 430.5 nm. We studied also the latitudinal profile of the altitude determined as the transmittance T=0.5 at the limb with the use of the plate scale. The results and the consideration for the cloud particles in the Haze region will be reported.  

6/11 (Wed) 16:30- 5F会議室 今村剛(准教授)
Morphology of Venus clouds at visible wavelengths  
Venus' clouds are considered to be almost featureless at visible wavelengths. This is because no notable absorption occurs in this wavelength region and the clouds are so thick that the cloud albedo is almost saturated. However, careful analysis of Venus Express/VMC images in the visible channel (513 nm) shows existence of various contrast features such as the dark polar oval and mottled clouds in the middle/low latitude. These features imply the importance of unknown ‘visible’ absorber and cloud dynamics in determining the visible albedo that is the key parameter in the climate system.  

6/4 (Wed) 16:30- 5F会議室 杉山耕一郎(招聘研究員)
Preliminary results of Mars surface environment assessment for a exploration program using numerical models  
To support designing landing module and observation instruments of a next Mars exploration program of JAXA, we try to assess the Mars surface environment using numerical models. We perform numerical simulations using a global circulation model (DCPAM) and find that Viking and Mars Pathfinder meteorological data are well reproduced by using DCPAM data with systematic calibrations. Based on these results, assessment of surface environment at Newton crater, which was a possible landing site, is performed to evaluate plausible range of meteorological conditions. In addition, in order to improve our assessment, a regional meteorological model (CReSS) and a large eddy simulation model (SCALE-LES) are now applied to Mars for evaluation of variability associated with mesoscale and boundary layer scale disturbances. In this seminar, we present the assessment using DCPAM in detail and also preliminary results using CReSS and SCALE-LES.  

5/21 (Wed) 16:30- 5F会議室 Lee Yeon Joo(招聘研究員)
Long-term temporal variations of UV reflectivity of Venus observed by the Venus Monitoring Camera onboard Venus Express  
The UV channel of the Venus Monitoring Camera (VMC) onboard Venus Express (VEX) detects dark and bright features at the cloud top level all over the globe. This UV contrast is affected by the abundance of an unknown UV absorber, which is located within the upper cloud layer, and the upper haze above the cloud tops (Pollack et al.,1979; Esposito, 1980). The unknown UV absorber is a major sink of solar energy in the Venus middle atmosphere (Crisp, 1986). The upper haze and clouds take part in sulfur photochemical processes in the Venus mesosphere (Mills et al., 2007). At the cloud top altitude the zonal wind speed is highest, resulting in changes in cloud morphology in a few days. Therefore, the features shown in the UV images are diagnostic for atmospheric dynamics and chemistry. By analyzing VMC UV images, we found there is a clear decreasing trend of the global mean albedo by 20-30% over 2000 orbits (=2000 Earth days) of VEX operation. This decrease is driven by changes at high latitudes. This implies that the typical latitudinal albedo distribution, bright polar hood and dark equatorial region, varies over time. The latitudinal difference in albedo changes from a clear brightness gradient from pole to equator to an almost identical brightness in both regions. Interestingly, this temporal variation is similar to that of the SO2abundance above the cloud tops, observed in the same period (Marcq et al., 2013). This suggests a reduction of SO2 over the equator decreases the amount of upper haze at high latitudes, as less sulfur is supplied by the meridional circulation. We investigate the phase angle dependence of the latitudinal albedo difference, which reveals that the vertical distribution of the UV absorbers and the upper haze varies in time as well. Our results show large scale variations in Venus’ atmospheric dynamics near the cloud tops, represented by temporal changes in the amount of upper haze at high latitudes and/or in the vertical distribution of the unknown UV absorber.  

5/7 (Wed) 17:15- 5F会議室 佐藤毅彦(教授)
Development of a dust imager for Mars landing mission  
We report progress in developing a dust imager for future Mars landing missions. As Martian dust is a key element of its environment and a potential hazard for human exploration, it is essential to know what is Martian dust and how it works. However, little is known about the Martian dust due primarily to lack of measurements. Direct imaging would greatly increase our knowledge about the Martian dust (previously, an Atomic-Force Microscope onboard Phoenix acquired just one image).
The dust imager under development is not a microscope but a "bare" imaging sensor of which pixels are fine pitched. After exposing the sensor to the air with dust for a while, we illuminate the sensor with a parallel beam so that shadows of particles on the sensor are directly imaged. In this way, the imager does not need a focusing mechanism and is expected to be very light-weighted and robust. Although the status is still the laboratory-experiment level, this small tool would greatly contribute to the Mars science and exploration.  

4/23 (Wed) 16:30- 3F会議室 樫村博基(プロジェクト研究員)
Energy spectra of atmospheric motions simulated by a high-resolution general circulation model of Venus  
We have developed a simplified Venus version of AFES (Atmospheric GCM for the Earth Simulator) and performed a very high-resolution simulation of the Venus atmosphere. In this seminar, we report and discuss kinetic energy spectra obtained from the high-resolution simulation. The model resolution is T159 (i.e., about 0.75 deg x 0.75 deg grids) and L120 (Δz is about 1 km). In the model, the atmosphere is dry and forced by solar heating with diurnal change and Newtonian cooling that relaxes the temperature to a zonally uniform basic temperature which has a virtual static stability of Venus with almost neutral layers. A fast zonal wind in a solid-body rotation is given as the initial state. Time-integrations are performed until the solution achieves a statistically steady state. We calculate horizontal kinetic energy per unit mass per unit wavenumber. The energy decreases by -5/3 power law in a range from wavenumber 4 to 45. Both in lower and higher wavenumber sides, the energy shows higher decreasing rate. We decompose the horizontal kinetic energy into the rotational and divergent components for a further analysis. Comparison with Earth’s energy spectra and that with power spectra of Venus cloud brightness are also discussed.  

4/16 (Wed) 16:30- 6F会議室 安藤紘基(プロジェクト研究員)
The proposal of the new Venus exploration  
As the mechanisms to drive and maintain the Venusian super-rotation, it is thought that angular momentum transports by meridional circulation or atmospheric waves play an important role. Then, it is essential to investigate the following two items from observation:

・ Structure of the meridional circulation
・ Three dimensional structures of atmospheric waves

It is expected that the former can be examined by analyzing image data obtained in Venus Express or Akatuki. On the other hand, the horizontal structures of the waves are gradually clarified by the analysis of image data obtained in Venus Express. In addition, the five cameras onboard Akatsuki will also give us useful information about them in the future. However, it is difficult to examine the vertical structures of the waves exactly by imaging measurements. One of the most suitable methods to examine the vertical structures of the waves is radio occultation technique, which enables us to retrieve the vertical temperature profiles with high precision and high resolution. However, there are two problems in the previous measurement styles as follows:

・ The observation chance is limited by the positions of the Earth, Venus and a spacecraft.
・ The place which can be observed is restricted by the orbit of a spacecraft.
・ Basically we can obtain only two temperature profiles per day.

Thus we cannot observe the short period waves or small-scale ones, which prevents us understanding the wave structures completely. Now JAXA has a plan to improve Epsilon rocket to bring a small spacecraft to Venus or Mars, and it is necessary to consider a next-generation planetary science mission. In this seminar, I am going to make a presentation about the new radio occultation measurement mission which we are devising now.